Moving aircraft on the ground between landing and takeoff without using the aircraft's main engines or tow vehicles has been proposed, and approaches for achieving the benefits that accompany such autonomous aircraft ground travel are being investigated. Non-engine drive means mounted to drive one or more aircraft landing gear wheels have been proposed. Such non-engine drive means may be electric, hydraulic, or pneumatic, although most effort has focused on developing electric drive means systems capable of integration into an aircraft wheel to drive the wheel and, therefore, the aircraft during taxi. An aircraft's auxiliary power unit (APU) is the preferred source of electric power supplied to landing gear wheel drive means for what are being referred to as electric taxi systems. The significant fuel and other savings found to accompany the autonomous aircraft ground movement possible when aircraft can move efficiently during ground travel without reliance on the main engines make electric taxi systems very attractive.
Electric and other drive means can be mounted on an aircraft to power nose landing gear wheels, main landing gear wheels, or both nose and main landing gear wheels. For example, in U.S. Pat. No. 7,445,178 to McCoskey et al, electric motors are described mounted on nose landing gear wheels, while in U.S. Patent Application Publication No. US2012/0104159 to Charles et al, electric motors are described to be mounted on main landing gear wheels. The direction of ground travel is typically controlled in a taxiing aircraft by a steering system in the nose landing gear that turns the nose wheels in a desired travel direction. Available aircraft steering systems, however, are used on aircraft that rely on the operation of one or more of the aircraft's main engines to move the aircraft during ground travel.
Locating an electric or other drive means in one or more aircraft nose landing gear wheels does not impact operation of a nose wheel steering system. Typically, aircraft accomplish steering by swiveling a lower portion of a shock strut supporting the nose landing gear wheels. A hydraulic steering unit is usually mounted on a fixed portion of the shock strut and is linked to a swiveling portion of the nose landing gear structure to which the nose wheel or nose wheels are attached. A drive means mounted within a nose wheel does not interfere with this steering action. A nose wheel-mounted drive means can be operated in conjunction with the steering system to maneuver the aircraft effectively through a range of different kinds of turns.
A drive means mounted to drive one or more main landing gear wheels cannot presently operate to maneuver a turning aircraft as effectively, however. An aircraft with drive means mounted on two main landing gear wheels that must make a tight turn will experience one driven main wheel turning while the other driven main wheel is not turning. The main wheel drive means, by themselves, cannot determine whether the nose landing gear has turned to turn the aircraft as needed. The main wheel drive means could be spinning the aircraft on its axis and/or causing the nose wheels to skid. If the distance travelled by each of the main landing gear drive means is the same, the nose landing gear will be moving in a straight line, whether the nose wheels are straight or turned. The resulting side loads on the nose landing gear structures in a turned nose gear could cause these structures to be fatigued, to weaken, or even to be irreparably damaged. Consequently, some type of steering control for the nose landing gear is required when drive means are to be mounted to drive an aircraft's main landing gear wheels during taxi.
The self-contained taxi system described by Kelly et al in U.S. Pat. No. 3,807,664 includes a hydraulic mechanism connected to an aircraft's main wheels that controls wheel drive speed and torque to drive aircraft wheels at taxi speed and an aircraft's electro-hydraulic steering system to control nose wheel steering during taxi. Control of aircraft movement and nose wheel steering is accomplished primarily by regulating hydraulic fluid flow. An optional separate hydraulic nose wheel drive mechanism is also provided to drive the nose wheels and control nose wheel turns separately and together with the control of the main wheels by regulating pressure of hydraulic fluid, although electric control is also contemplated. The kind of control required to avoid loads on the nose gear described above is not suggested.
There is a need for a system for controlling simultaneously aircraft non-engine taxi and nose wheel steering in an aircraft with non-engine drive means mounted on main landing gear wheels to effectively drive the aircraft on the ground and maneuver the aircraft through any turns the aircraft is required to make during ground movement.